1. Field of the Invention
The invention relates to an audio signal encoding apparatus and an encoding method and an audio signal decoding apparatus and a decoding method which are applied to transmit a digital audio signal by, for example, a digital satellite broadcasting.
2. Description of the Related Art
A powerful error correction code is used for transmission of information in a digital satellite broadcasting. Therefore, even if a C/N ratio of a transmission path deteriorates to a certain degree, information of substantially the same quality as that of a transmission signal can be received by error correction.
However, if the C/N ratio suddenly deteriorates as in case of a rainy day and if an error generated exceeds error correcting ability which the error correction code has, the quality of reproduced information suddenly deteriorates to such an extent that the contents cannot be understood. On the other hand, in case of an analog broadcasting, a quality of reproduced information gently changes with the decrease in C/N ratio. FIG. 1A conceptually shows the foregoing difference between the digital broadcasting and the analog broadcasting in the case where an axis of abscissa shows an error rate and an axis of ordinate indicates a quality of data that is reproduced. In case of the analog broadcasting, the quality gradually deteriorates as shown by a broken line. In case of the digital broadcasting, the quality suddenly deteriorates as shown by a solid line. Actually, even in case of the digital broadcasting, characteristics (graceful degradation) such that the quality of the reproduction data gradually deteriorates as in the analog broadcasting are preferable.
The transfer of audio data on a network will now be considered. Even if the signal processing abilities on the encoder side and the decoder side have abilities enough to execute real-time processes, when the network is busy and an enough data transfer amount cannot be assured, a reproduction signal cannot be obtained in a real-time manner. In such a case, since a decoding process cannot be performed until a predetermined amount of data is accumulated on the decoder side, a reproduction sound becomes an intermittent sound and a reproduction sound quality remarkably deteriorates. Further, even when data processing ability on the decoder side connected to the network is inadequate, the quality of the reproduction audio sound similarly deteriorates.